It is known that tetraazaindenes and their salts represented by the general structural formula ##STR4## wherein M is a metal cation selected from Group IA and IIA metals and y is the valence of the metal cation, are useful as stabilizers in photographic emulsions. For example, it is known that in a silver halide emulsion there is a detectable amount of the silver salt reduced during development in the unexposed areas. The result can be degradation of the photographic image. It is well known that the presence of tetraazaindenes in the silver halide emulsion can decrease the degradation of the developed photographic image. Accordingly, there has been considerable effort expended in the art to prepare tetraazaindene compounds.
Prior workers have shown that the preparation of tetraazaindenes is not an easy task. Several approaches have been taken by prior workers to overcome this problem.
One prior art approach ("prior art approach 1") to prepare tetraazaindenes is by the condensation of a .beta.-keto ester, a malonic ester or a mononitrile of a malonic ester, with a 3-amino-1,2,4-triazole. One problem with this approach is that preparation of the .beta.-keto ester for the reaction can be inefficient in regard to yield and the .beta.-keto ester can decompose and consequently result in a reduced overall reaction product yield. Another problem is that the time involved for the reaction to yield a commercial amount of tetraazaindene is lengthy and thus involves a significant manufacturing process cost. A third problem is that undesirable intermediate compounds that are unstable and difficult to isolate are formed, necessitating an additional purification step or steps and negatively effecting the process times, cost, and yield.
Another prior art approach ("prior art approach 2") is to condense a triazole or a polyazole having at least one primary amino group with a .beta.-keto ester, a .beta.-keto acetal, a cyclic .beta.-keto ester, or a malonic or cyanacetic ester. This approach likewise has all the above-stated problems that can result in increased process times and costs and decreased yield of tetraazaindenes.
A third prior art approach ("prior art approach 3") results in a tetraazaindene having a carboxyl group by deesterification of the tetraazaindene, said tetraazaindene having been prepared by condensing an alkoxymethenemalonic acid ester with a 3-amino-1,2,4-triazole compound under alkaline conditions. The free acid form is obtained by acidification of the deesterified tetraazaindene. This process not only has the above-described problems of the prior art processes but it involves the additional process step of deesterification followed by acidification, thus further increasing the process time, process steps, and process costs.
This invention solves the prior art problems noted above. It does not generate unstable and difficult to isolate intermediate compounds. Instead, intermediates are produced that react faster and more completely, thus eliminating the costly and time-consuming step or steps of isolation and purification associated with unstable intermediate compounds.
Furthermore, the reaction conditions of this invention are milder and result in increased yield of the tetraazaindene. Also, the reactions take place faster and thus involve decreased process costs. Thus, by means of this invention, there is provided an improved, three-step synthesis of tetraazaindenes that solves the above-stated prior art problems.